Currently, we have 3 colors within our subtitles (If you do not count gray). Purple for the unencountered words, Green for known, and Orange for learning. I would like to suggest that when a word is marked green or orange, other forms of that word can be auto-tagged in another color, for example, blue. The blue will signal to the user to pay some extra attention to this word as they are learning it in another form or have signified that they know it already in another form. I believe this is beneficial to the user studying in that it will encourage them to dissect new words and boost pattern recognition abilities in their target language.
Example: My target language is Russian and I encounter друзья (friend). I know this word already so I mark it as known and it is green now. Later I come across the word друзьями (Friends in instrumental case) maybe it throws me off because I don’t know the instrumental case well, or hadn’t come across this word in this case and never thought about it. But I see the word is blue, so I look for a minute, and either I can recognize the root “друзь” or maybe I don’t make the connection. But I thought about it, I click the word, get the definition, let the gears click, and mark that form for learning.